The concept Modular Multilevel Converter (M2C) denotes a class of Voltage Source Converter (VSC). It has one or several phase legs connected in parallel between two DC bars, a positive DC+ and a negative DC−. Each phase leg consists of two series-connected converter arms. The connection point between the converter arms constitutes an AC terminal for the leg.
Each arm consists of a number (N) of submodules. Each submodule has two terminals. Using these terminals the submodules in each arm are series-connected so that they form a string. The end terminals of the string constitute the connection terminals of the arm. By controlling individual modules in each arm, a voltage corresponding to the accumulation of insertion voltages can be provided on the AC terminal.
Such a converter is known from DE10103031. In this document, a method to equalize the voltages in the submodule capacitors within the arm is briefly described. For each arm, a modulator determines when the number of inserted submodules shall change. The principle for equalizing is that, at each instant when a change of the number of inserted submodules is commanded, a selection mechanism chooses the submodule to be inserted or bypassed depending on the actual current direction in the arm (charging or discharging) and the corresponding available submodules in the arm (bypassed highest voltage/bypassed lowest voltage/inserted highest voltage/inserted lowest voltage). Such a selection mechanism aims to achieve that the DC voltage across the DC capacitors in the submodules are equal, uC,SM(t).
A problem with the prior art is the presence of a circulation current going through the legs between the DC terminals.